You can find information on how to setup the Proxy on the ET1 using Android v4.1.1 on the Integrator Guide:
link on the support site:
Direct link to the document
Take a look at chapter 5.4, Configuring for a Proxy Server.
However the document does not mention how to set the credentials, but I don't have a setup to check out what happens in that case (or a device to see if the options are there if you tap the menu).
I saw it but the real issue is for credential
I have read on the web that seems Android OS has not integrate possibility to enter credential.
For that it seems that you need to use a third part application